When a heating operation is performed in an air-conditioning apparatus, outside air with a high humidity causes frost formation in an outdoor heat exchanger. When frost formation occurs, a ventilation passage of the outdoor heat exchanger is narrowed, with the result that a flow rate of the outside air circulating through the outdoor heat exchanger is reduced. When the flow rate of the circulating outside air is reduced, a heat exchange amount is reduced. To compensate for the reduction of the heat exchange amount, correspondingly, an evaporation temperature of refrigerant flowing through the outdoor heat exchanger is lowered. When the evaporation temperature of the refrigerant is lowered, a surface temperature on the outside air side of the outdoor heat exchanger is also lowered, and frost formation is more liable to occur, with the result that frost formation becomes significant. When the situation remains unchanged, an amount of heat received by the outdoor heat exchanger from the outside air is reduced, and consequently, an amount of heat rejected from the indoor heat exchanger is also reduced. For this reason, in a related-art air-conditioning apparatus, when frost formation occurs in the outdoor heat exchanger, defrosting of the outdoor heat exchanger is performed.
As such a related-art air-conditioning apparatus configured to perform the defrosting of the outdoor heat exchanger, there has also been proposed an air-conditioning apparatus configured to perform the defrosting of the outdoor heat exchanger while performing the heating operation to prevent degradation in comfort in a room due to stoppage of the heating operation during the defrosting operation (see, for example, Patent Literature 1 and Patent Literature 2). Specifically, in the air-conditioning apparatus described in Patent Literature 1 and Patent Literature 2, an outdoor heat exchanger is constructed by a plurality of flow passages that are connected in parallel. Further, the air-conditioning apparatus described in Patent Literature 1 and Patent Literature 2 are configured to perform the defrosting of the outdoor heat exchanger while performing the heating operation in the following manner. During the heating operation, high-temperature gas refrigerant discharged from the compressor is caused to flow through a part of the flow passages, and the flow passage through which the high-temperature gas refrigerant discharged from the compressor is caused to flow is sequentially switched.